[Cayenne-dev] server-to-client event push - Re: relationship query and cache refreshing in 3tier cayenne

I just took a glance at the early draft of JSR-315 (and looks like
Jetty 7 early releases already have this API built in).

I *hope* we can emulate a persistent connection with that, using a
protocol similar to this:

1. Client sends a "give me events" request in the beginning of the
session (separate from data requests), and reads everything that comes
back.
2. Server suspends the request thread, and keeps it suspended until
the client quits.
3. When an event arrives on the server, it is written to response and
response.flushBuffer() is called (I guess we'll need to resume the
request to do that and then suspend it again ... I hope multi-suspend
is supported ? Or can we flush without resuming?)

Of course all that is pluggable via an EventBridge implementation, so
given the time and motivation we can download Jetty 7 and start
playing with it right away.

Andrus

Three thoughts:

1. Is your idea to have one queue per client, or a single queue with
some sort of sequence numbering so a client can grab events later than
a particular timestamp/sequence?

2. Even without continuations the basic idea could still work if the
client received these events attached to query results. So every time
a client performs a query (or executes a special polling request)
these events are received. Not quite so real-time, but possibly useful
in many circumstances.

3. Are the events here just refresh query cache events, or are we
talking about 'invalidate record id 1234 from entity Artist' messages?
Possibly that will generate too much traffic since the server will not
know which clients have those entities already cached and will have to
propagate all invalidate messages to all clients. I'm still a little
hazy about whether the server has a copy of the same contexts as the
client does, so perhaps it does know which records the client is
holding, but I'm not sure how SHARED_CACHE impacts that.


Ari



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As the events will be dispatched immediately (there will be no client
polling), the only queue involved is the local EventManager queue.
There is a single one per DataDomain and it is shared by all server
contexts.
Yeah - that's another possible mechanism. The mechanics of it will be
different. Unlike continuations approach, this will require a "delayed
client queue".
Both types of events... (I guess each type can be suppressed or
enabled via configuration). I forgot to mention one thing though - we
already have per-object events (since Cayenne 1.1), but there is no
query cache events (any such events are specific to an implementation
of the QueryCache, and are not a part of Cayenne). So that's something
that we need to design.

Yes - the server contains "peer" contexts of all client contexts,
stored in HttpSession. Their state is almost the same as client's, the
only difference being objects loaded during callbacks execution.

Andrus